Properties such as fiber moisture content, resin content, optical characteristics and fiber characteristics, such as shives and fiber length distribution, are generally known to affect the quality and properties of the finished fiberboard. Appropriate and, in particular, a stable fiber moisture content facilitates, e.g. checks on the pressing phase of panel fabrication and creates conditions for good control of the density profile of these panels.
After the panel is pressed, the resin added during panel fabrication hardens and creates, with the fiber network, a strong fiber composite structure. The resin is added either by injection into the blower line after the defibering process or into a mechanical resin mixer after the drying phase. The resin content has a major impact on the finished panel's strength properties, such as bending strength and tensile strength. The resin content is often governed by the specific properties the finished panel is to have. However, an excessive amount of resin is often dispensed in order to ensure the desired level of quality, as there is no way to accurately control other factors, such as fiber moisture, fiber length distribution and density, factors which also affect the properties of the finished fiberboard.
To date, the way in which fiber properties (such as the fiber length distribution and shives content) should be specified in order for the fiber network to optimally achieve the desired properties for the finished fiberboard has been unclear. One reason for this is because of the previous unavailability of any simple and rapid measurement method for determining and characterising fiber length distribution for MDF (medium density fiberboard) fibers. However, equipment is now available for determining and characterising fiber properties with the aid of image analysis. This is a complex technique, however, which is only practical in a laboratory environment, and which is definitely unsuitable for on-line measurement in the fabrication of MDF.
Since no facilities have hitherto been available for continuous on-line measurement of parameters for fiber characterisation during fiberboard fabrication, studying the way in which fiber properties should be devised in order to manufacture finished panels with the desired properties in an optimal fashion has been very difficult. This also applies to efforts to achieve optimal control of the defibering process so that the desired fiber properties are attained.
The need for resin can be minimized, and the production costs reduced, when fiber properties are controlled. Resin accounts for about one-third of the direct cost of fiberboard fabrication. Resin coating, i.e. the distribution of resin on the wood fibers, is also affected by the distribution of fiber lengths. Fine fiber fractions require more resin than the thicker fibers. As a result, increasing the resin content does not produce the anticipated increase in the strength of the finished panel when the fiber length distribution of the fibers used is inappropriate.
Multivariate analysis of spectral data for determining wood chip components, such as Klason lignin, extract and the total amount of carbohydrates, is described in “Dialog Information Services,” File 248, PIRA, Dialog accession No. 00393878/5, PIRA accession No. 20017450, Meder R. et al.: “Prediction of wood chip and pulp and paper properties via multivariate analysis of spectral data,” Melbourne, Australia, 2–6 May 1994, pp. 479–484, which is incorporated herein by reference thereto. In this article, Principal Component Analysis (PCA) and Principal Component Regression (PCR) were used for analysing near-infrared (NIR) spectra, Fourier-transformed infrared (FTIR) spectra and nuclear magnetic resonance (NMR) spectra taken from examined wood chips.
International Patent Application No. WO 97/04299 describes multivariate data analysis of near-infrared (NIR) spectra taken from raw materials, such as sawdust, shavings and wood chips, used for manufacturing chip board. Use of the measured values for controlling panel fabrication is also described. One of the objectives of the present invention is to refine this technique, based on multivariate analysis of spectral data, so it can also be used for continuous determination of the properties of wood fibers for fiberboard fabrication, these properties being of decisive importance to the properties of the finished panel.